1. Field of the Invention
The present invention relates to a voltage regulator having improved power supply rejection ratio (PSRR) characteristics while maintaining a narrow response band.
2. Description of the Related Art
Voltage regulators have been incorporated in mobile stations such as mobile telephone sets or electronic notebooks which need to be small both in size and power consumption.
In a first prior art voltage regulator (see: FIG. 2 of JP-10-260741-A), a reference voltage generating circuit generates a reference voltage. A drive transistor is connected between a power supply terminal and an output terminal and has a control terminal. A voltage divider generates a feedback voltage which is an intermediate voltage between voltages at the output terminal and the ground terminal. A differential amplifier generates an error voltage in accordance with the feedback voltage of the voltage divider and the reference voltage, and transmit it to the control terminal of the drive transistor. An oscillation preventing capacitor is connected between the control terminal of the drive transistor and the output terminal. This will be explained later in detail.
In the above-described first prior art voltage regulator, since the circuit current of the differential amplifier is relatively small and the capacitance of the oscillation preventing capacitor is relatively large, the response band is so narrow that the operation is stable. However, if a high frequency noise higher than a predetermined value is applied to the power supply voltage, the PSRR characteristics deteriorate rapidly, so that such a high frequency noise cannot be compensated for by the negative feedback control. As a result, such a high frequency noise would appear at the output terminal.
In the above-described first prior art voltage regulator, in order to improve the PSRR characteristics at a higher frequency, one approach to is increase the circuit current of the differential amplifier, and another approach is to decrease the capacitance of the oscillation preventing capacitor. In this case, however, the response band is also broadened, so that the operation would be unstable. Also, the former approach would increase the power consumption.
In a second prior art voltage regulator (see: JP-2001-159922-A), differential amplifiers (operational amplifiers) are added to the elements of the above-described first prior art voltage regulator. This also will be explained later in detail. As a result, the amplification of a differential amplifier section formed by the differential amplifiers is increased to improve the PSRR characteristics.
Even in the above-described second prior art voltage regulator, however, the response band would be broadened. Also, since the number of differential amplifiers (operational amplifiers) is increased, the power consumption would be increased and the circuit size would be increased.